On April 15, 2010 I was birding at Horn Pond and was watching a grackle gathering nesting material.
It landed on the back of this broken off stump - but it didn't reappear for several minutes. Very strange.

Afte a great deal of slogging through the mud I was able to get a view of the back side of the stump. I crouched down to minimize my threat posture, focused thecamera on the hole, and was rewarded by seeing the grackle exiting the nest hole. I was astonished, but looked it up in Birds of North America and it said "Unusual nest sites include birdhouses, tree cavities (e.g., old woodpecker cavities), buildings (e.g., barns), and occupied nests of Osprey  and Great Blue Heron"

It will be fun to follow these birds to see if they successfully rear young.